We have evaluated the reflex and the mechanical torque generated by the animal in two ways. On the one hand, we relied upon modelling of the neck-head apparatus, on the other, we assessed the role of the reflex loop by comparing head movements before and after surgical interruption of the afferent input subserving the hypothesized servo-action (section of cervical dorsal roots). Our results indicate that the lond compensating capacities due to the reflex segmental and suprasegmental mechanisms stimulated by the application of an opposing force ranged from 10% to 30% in different aaimals and that a larger amount of compensation was due to the viscoelastic properties (mechanical) of the neck musculature. However, the combined reflex and the mechanical extra torque never provided full compensation. BIBLIOGRAPHIC REFERENCES: Bizzi, E.: Motor coordinations: central and peripheral control during eye-head movement. In: M.S. Gazzaniga & C. Blakemore (Eds.), Handbook of Psychobiology (1975) Academic Press: New York, pp. 427-437. Bizzi, E., Polit, A. and Morasso, P.: Mechanisms underlying achievement of final head position. J. Neurophysiol., 39: 1976, 435-444.